The present invention relates to the field of medical ultrasound imaging, especially blood flow imaging in living organisms.
An ultrasound imaging system scans a human body with ultrasound beams to create two-dimensional images of organs such as the heart, a fetus, the liver, or sometimes blood vessels (e.g. carotid arteries). This ultrasound image mode is called a B-mode image, which is a grayscale image. In addition to the B-mode image, today""s high performance ultrasound imaging systems can detect and display blood flow. Because blood flow provides important physiological information to clinicians, it is desirable to display blood flow and its physiological state. Blood flow is usually displayed as a color image on top of the grayscale image (or B-mode) of vessel structures, such as vessel walls, bifurcations, or sometimes lesions. This mode is usually called color flow or color Doppler if the blood detection involves the Doppler technique. More quantitative measurements of blood flow are usually performed with the spectral pulsed-wave (PW) Doppler.
The sound pressure level of an ultrasound signal scattered from blood is much lower (usually xe2x88x9220 to xe2x88x9240 dB lower) than that scattered from tissue. Therefore, the signal-to-noise ratio (SNR) in an ultrasound signal is critical where color flow or color Doppler detection is used. Because SNR is critical in color flow and color Doppler detection, many efforts have been made to increase the signal to noise ratio in the ultrasound signal and to increase sensitivity in detecting blood flow. One such method is to use a narrower band ultrasound signal for color flow or color Doppler detection than that used for tissue detection. An additional integrator in the axial direction may also be used to further increase the SNR. Unfortunately, reducing the bandwidth of the ultrasound signal and increasing the number of integrators in the axial direction will both limit spatial (or axial) resolution substantially. As a result of these methods, color flow and color Doppler imaging have several times lower spatial resolution than B-mode tissue imaging does. For certain applications, such as the early detection of certain vascular diseases, the reduced spatial resolution associated with color flow and color Doppler imaging is not ideal. In addition, since blood flow is usually displayed in colors rather than grayscale, the blood flow image is usually superimposed on the grayscale image of blood vessel walls. Unfortunately, the blood flow image often overwrites the image of the vessel walls, making the borders between vessel walls and blood unclear.
A method and apparatus of blood flow imaging is disclosed. The method comprises receiving a beamformed signal indicative of an ultrasound signal reflected from a target within a body. Based upon the beamformed signal, imaging tissue within the body is performed, generating thereby a tissue signal. In addition, based upon the beamformed signal, detecting blood flow within the body is performed, generating thereby a blood flow signal. The tissue signal and the blood flow signal are combined to generate a composite image of tissue and blood flow.